1143. Lowest Common Ancestor (30)

1143. Lowest Common Ancestor (30)

#include <bits/stdc++.h>
using namespace std;
struct TreeNode
{
    int val;
    TreeNode *left,*right;
    TreeNode(int v):val(v),left(NULL),right(NULL){}
};
int pre[10000+32],ino[10000+32];
TreeNode * build(int preL,int preR,int inoL,int inoR)
{
    if(preL>preR) return NULL;
    int e=pre[preL],idx=inoL;
    while(idx<=inoR&&e!=ino[idx]) ++idx;
    TreeNode *r=new TreeNode(e);
    r->left=build(preL+1,preL+1+idx-1-inoL,inoL,idx-1);
    r->right=build(preR+idx+1-inoR,preR,idx+1,inoR);
    return r;
}
TreeNode *search(TreeNode *r,int e)
{
    while(r){
        if(e<r->val) r=r->left;
        else if(e>r->val) r=r->right;
        else return r;
    }
    return r;
}
TreeNode *LCA(TreeNode *r,TreeNode *px,TreeNode *py)
{
    while((r->val-px->val)*(r->val-py->val)>0){
        r=(r->val<px->val)?r->right:r->left;
    }
    return r;
}
int main()
{
    int m,n;
    scanf("%d %d",&m,&n);
    for(int i=0;i<n;++i){
        scanf("%d",pre+i);
        ino[i]=pre[i];
    }
    sort(ino,ino+n);
    TreeNode *r=build(0,n-1,0,n-1);
    while(m--){
        int x,y;
        scanf("%d %d",&x,&y);
        TreeNode *px=search(r,x),*py=search(r,y);
        if(!px&&!py) printf("ERROR: %d and %d are not found.\n",x,y);
        else if(!(px&&py)) printf("ERROR: %d is not found.\n",px?y:x);
        else{
            TreeNode *res=LCA(r,px,py);
            if(res==px) printf("%d is an ancestor of %d.\n",px->val,py->val);
            else if(res==py) printf("%d is an ancestor of %d.\n",py->val,px->val);
            else printf("LCA of %d and %d is %d.\n",px->val,py->val,res->val);
        }
    }
    return 0;
}